Metal plating process

ABSTRACT

A PROCESS FOR APPLYING AN ALUNINUM PLATE ON A WIDE VARIETY OF MATERIALS TO PRODUCE ARTICLES HAVING DESIRABLE QUALITIES EXEMPLIFIED BY GOOD CORROSION RESISTANCE. A FIRST EMBODIMENT OF THE INVENTION IS CHARACTERIZED BY CONTACTING THE SUBSTRATE TO BE COATED WITH A SOLUTION CONTAINING A SUITABLE CATALYST AND A TRIBYDROCARBYLAMINE COMPLEX OF ALUMINUM HYDRIDE AT RELATIVELY LOW TEMPERATURES, I.E., BELOW 150*C. DECOMPOSITION OF THE COMPLEX IS EFFECTED BY CATALYSTS AND ALUMINUM PLATING OCCURS ON THE EXPOSED SURFACES OF THE SUBSTRATE. IN A SECOND PROCESS EMBODIMENT THE SUBSTRATE IS COATED WITH A CATALYST AND SUBSEQUENTLY BROUGHT INTO CONTACT WITH A SOLVENT CONTAINING A DISSOLVED ALUMINUM HYDRIDE-TRIHYDROCARBLAMINE COMPLEX TO EFFECT THE DESIRED ALUMINUM DEPOSITION. THE PROCESS OF THIS INVENTION PROVIDES A HIGHLY USEFUL AND NOVEL METHOD OF PLATING SUBSTANTIALLY ANY MATERIAL WITH A UNIFORM, STRONGLY ADHERENT ALUMINUM COATING OF DESIRED THICKNESS.

United States Patent 3,705,051 METAL PLATING PROCESS Paul Kobetz, Warren E. Becker, and Albert P. Giraitis, Baton Rouge, La., assignors to Ethyl Corporation, New York, N.Y.

No Drawing. Continuation of abandoned application Ser. No. 705,016, Feb. 13, 1968. This application Dec. 10, 1970, Ser. No. 96,989

Int. Cl. B44d 1/02 U.S. Cl. 11747 H 19 Claims ABSTRACT OF THE DISCLOSURE A process for applying an aluminum plate on a wide variety of materials to produce articles having desirable qualities exemplified by good corrosion resistance. A first embodiment of the invention is characterized by contacting the substrate to be coated with a solution containing a suitable catalyst and a trihydrocarbylamine complex of aluminum hydride at relatively low temperatures, i.e., below 150 C. Decomposition of the complex is eifected by catalysts and aluminum plating occurs on the exposed surfaces of the substrate. In a second process embodiment the substrate is coated with a catalyst and subsequently brought into contact with a solvent containing a dissolved aluminum hydride-trihydrocarbylamine complex .to eifect the desired aluminum deposition. The process of this invention provides a highly useful and novel method of plating substantially any material with a uniform, strongly adherent aluminum coating of desired thickness.

This is a continuation of application Ser. No. 705,016, filed on Feb. 13, 1968, now abandoned.

This invention relates to a process for plating aluminum metal on various materials by contacting these materials with a catalyst and a tr'ihydrocarbylamine complex of aluminum hydride to produce articles having enhanced physical and chemical properties.

Heretofore, considerable interest has developed in the field of metal coatings application, and particularly in the area of plating with aluminum metal. Much of the emphasis placed on aluminum plating is due to the fact that aluminum provides a desirable protective coating for oxidizable metals since the surface of an aluminum coating exposed to air rapidly forms an oxide film which protects the remainder of the coating and the metal substrate from oxidation. Another growing use for aluminum metal plating techniques finds application in the electronics industry where it is frequently desirable to coat a material such as plastic with a metal having good electrical conduction properties, as in the manufacture of aluminum coated semiconductor devices. Still another important application of considerable interest and importance is the aluminum plating of certain critical materials such as space capsule components and aircraft fasteners for use in the aerospace and aircraft industries.

In view of the extensive and growing need for aluminum plated materials, there have been many attempts to develop processes for effectively plating aluminum on metal substrates such as iron and steel as well as other materials, including plastics, glass, hardened clay, and the like. See, for example, U.S. 3,041,197 granted June 26, 1962, and Government Report ASD-TDR-62-907, entitled Vapor Plating of Aluminum on Steel by Mr. J. I. Crosby of the Aeronautical Systems Division, Wright- Patterson Air Force Base. Unfortunately, aluminum is a difficult metal to apply in the form of a coating. One difficulty of considerable consequence is the propensity of aluminum to diffuse into the interstices of metals and other materials being coated at moderate to high temperatures. This phenomenon frequently causes a weakening of the substrate and renders the application of relatively heavy aluminum coatings ditficult without sacrificing, to some degree, the strength of the material plated. A similar substrate weakening effect is dramatically illustrated Where aluminum coatings are deposited by techniques requiring the application of heat to plastic materials which have a tendency to deform or even melt at elevated temperatures. In this regard, it is significant that substantially all of the prior art processes, both physical and chemical, which disclose techniques for plating aluminum metal on various substrates are directed toward methods which utilize rather high temperatures. Of the chemical processes noted in the art, the majority of attention has been concentrated on vapor coating techniques wherein the plating of a substrate is effected by thermal decomposition of vaporous heat decomposable compounds of aluminum. Broadly, these processes disclose the heating of a decomposable aluminum compound to its vapor phase, transporting it into a plating chamber and introducing into the chamber a substrate preheated to a temperature above that of the vaporized aluminum compound. Under these conditions the aluminum compound decomposes and aluminum metal is plated on the substrate. See, for example, U.S. 2,921,868, granted Jan. 19, 1960. Another exemplary chemical process disclosing an aluminum plating operation in one of the lowest known temperature ranges is that noted in the above-mentioned U .8. Pat. No. 3,041,197, where the patentee indicates that the plating temperature of his process may range from 170 C. to 500 C. and in any event must be at least that at which the heat decomposable organic compound utilized, will decompose to liberate aluminum metal. Further, it is noteworthy that temperatures up to 1800 C. have been used in certain physical metal volatilizing techniques disclosed in the art. See U.S. 2,880,115, granted Mar. 31, 1959.

In addition to the problem of substrate fatigue presented by dilfusion of hot aluminum into heated substrate interstices, there are other disadvantages associated with the use of aluminum vapor plating and similar techniques requiring a relatively high temperature, i.e., over C. One such shortcoming which becomes apparent in vapor plating pnocesses is the requirement of an aluminum plating compound having the necessary degree of volatility without decomposition. A compound possessing these characteristics must be used in order that it may be vaporized for transportation and subsequent contact with a heated substrate for decomposition. Another disadvantage frequently encountered in vapor plating techniques is the necessity for maintaining a controlled atmosphere in the plating chamber; for example, the presence of oxygen in small amounts may cause the deposition of a highly oxidized aluminum coating. Further, the use of carrier gases, a necessity in vapor plating, may cause the formation of impure or alloy coatings on the substrate due to the presence of certain components in the gases. Additional disadvantages of vapor plating processes become apparent as a result of inefficient utilization of the plating compound due to large losses from aluminum deposition on the plating chamber interior and exhausting procedures. Still another problem is presented by the requirement that the vapor plating chamber be tightly sealed to prevent escape of the toxic aluminum plating compounds. A further shortcoming which is inherent in all of the existing aluminum plating processes which utilize high temperatures is the obvious equipment and instrumentation cost of heating and maintaining the various plating compounds and substrates to closely controlled temperatures in order to effect the desired aluminum deposition.

Accordingly, it is an object of this invention to provide a novel and useful process for plating aluminum metal on a wide variety of materials. Another object is to provide a process so characterized which is capable of depositing aluminum at temperatures significantly below those heretofore required to effect aluminum plating. A further object is to provide a simple process for effecting aluminum plating in a liquid medium rather than in the vapor phase where complicated and expensive equipment and procedures must be utilized. A still further object of the invention is to provide a process wherein a uniform and strongly adherent aluminum plating of desired thickness can easily be deposited without the necessity of reheating the substrate to be coated as was frequently heretofore necessary. Other important objects, features, advantages and characteristics of this invention will become apparent from the ensuing description and appended claims.

The foregoing objects and other features hereinafter emphasized are realized in two embodiments of this invention. Broadly, these embodiments disclose a novel and advantageous process for coating various materials with aluminum by facilitating contact among the material to be plated, a suitable catalyst, and a trihydrocarbylamine complex of aluminum hydride.

It has surprisingly been found that certain catalysts may be used to induce decomposition of aluminum hydride-trihydrocarbylamine complex compounds at relatively low temperatures. Further, this decomposition may be most expeditiously effected in solution to liberate aluminum according to hereinafter disclosed procedures. Thus, an extremely significant feature common to both embodiments of the invention is the requirement of process temperatures far below those necessary in the above noted and other previously developed aluminum plating techniques. At best, the older processes must be practiced at a temperature within the range from 170 C. to 500 C. as described in U.S. 3,041,197. In sharp contrast, it has been found that the plating process of this invention may be successfully carried out at room temperature or lower if sufficient time is allowed. However, it will be recognized that temperatures up to about 150 C. and higher may be utilized when it is desired to effect rapid aluminum plating, and in any event, the temperature should preferably be adjusted to at least 25 C. for practical operation. A more desirable operating temperature range for both embodiments of the inventive process is from about 25 C. to about 100 C., and a most preferred range is from about 60 C. to about 80 C. By plating at temperatures below 150 C. one avoids many of the undesirable effects of high temperature coating application, as heretofore discussed. The temperature of the plating system may be controlled by methods well known to those skilled in the art during the coating process in order to avoid excessive evaporation of plating constitutents. It will be appreciated that heat may be introduced into the system by preheating the substrate to be coated or applying heat to the plat-ing medium, externally, or both.

Another important feature of the invention is the discovery that the plating process may be carried out at substantially any desired pressure, although it is preferable to operate at atmospheric pressure for obvious purposes of convenience.

Still another advantageous feature found in both embodiments of this invention is the use of plating compounds which are readily decomposed at low tempera tures to effect aluminum deposition on a desired substrate. The catalyst-initiated decomposable aluminum compounds which may be employed in the process of this invention are, broadly, solvent-soluble or liquid trihydrocarbylamine complexes of aluminum hydride. The complex must be either a liquid or capable of being dissolved in a suitable solvent in order to form an effective medium in which decomposition and accompanying plating can occur, and it may contain impurities normally found in such compounds, such as Group I metals exemplified by sodium. Further, it has been discovered that the amine in the complex must be of the tertiary configuration. Thus, suitable plating compounds which may be utilized in the invention are tertiary aryl, cyclic, alkyl, alkenyl and aralkyl amine complexes of alumnium hydride, including the monoamine, diamine, triamine, etc., complexes. Typically, the amine component of these complexes may be triphenylamine; tetramethylethylene diamine; diphenylmethylamine; triethylenediamine; phenylmethylethylamine; tricyclohexylamine; hexamethylenetetramine; phenylcyclohexyloctyla'mine; or mixtures thereof, and other similar compounds which will readily decompose to liberate aluminum under proper conditions. A more preferred class of decomposable aluminum hydrideamine complexes for use in the invention are aliphatic tertiary amines, which include the trialkylamine and tri alkenylamine complexes of aluminum hydride. Further, the amines in these complexes may generally contain up to about 40 carbon atoms each, and preferably contain alkyl and alkenyl groups each having from 1 to about 10 carbon atoms. Thus, useful amines of this class are tri-n-butylamine; tri-sec-butylamine; bis-dibutylpentylamine; tri-tert-butylamine; n-butyloctyl-sec-butylamine; tripentylamine; bis-trihexylamine; trihexenylamine; trioctadecylamine; didecenylpentylamine; tridecenylamine; and the like, as well as mixtures thereof. A most preferred class of amines for use in the invention are those in the lower alkylamine complexes of aluminum hydride such as triethylamine, bis-triethylamine, triisopropylamine, and particularly, trimethylamine and bis-trimethylamine complexes of aluminum hydride. By the term lower is meant that the alkyl groups each con.-. tain 6 carbon atoms or less. The above compounds may be readily prepared by procedures well known to those skilled in the art.

Useful catalysts which have been found to be effective in both embodiments of the invention are generally, titanium halides, alkoxides and haloalkoxides, as well as other metal chlorides, exemplified by ferric chloride, manganese dichloride, titanium tetrachloride, titanium isopropoxide, and titanium dichlorodiisopropoxide. It is entirely possible that other suitable catalysts may be found on running appropriate screening tests. It will thus be appreciated that at present it is difficult, if not impossible, to exemplify every compound or complex which serves to facilitate the decomposition of trihydrocarbylamine complexes of aluminum hydride in this metal plating process. Based on available experimental results, titanium tetrachloride has proved to be a most preferred plating catalyst.

The materials or substrates which may be plated according to both embodiments of the invention hereinafter disclosed include substantially any object which possesses moderate structural strength at temperatures up to about C. and which is not significantly affected by contact with the plating compounds heretofore noted or solutions of these containing common organic solvents. Thus, materials such as iron, steel, brass, plastics (including films), glass, graphite, spun glass, ceramics, refractories, etc., may easily be platted by utilizing the inventive process.

Surface preparation of the material to be plated may be performed as a preliminary step prior to employment of the plating techniques of this invention. It has been found that many substrates need no such preparation while others exhibit an aluminum coating having improved adherence qualities when such preparation has been effected. Such methods as abrading (standing, etc.), and rough buffing may be used on plastics and similar substrates while acid etching is a favorite technique for metals. Other methods, including cleaning and degreasing, may also be used exclusive of or with the above preparatory procedures according to the knowledge of those skilled in the art.

In accordance with the first embodiment of this invention aluminum metal is plated on a substrate by contacting the substrate in an inert atmosphere with a liquid phase system or medium composed of a trihydrocarbylamine complex of aluminum hydride and a catalyst, a function of which is to induce decomposition of the aluminum hydride-trihydrocarbylamine complex. By inert atmosphere is meant the atmosphere surrounding the plating medium as distinguished from the plating medium itself. However, as hereinafter pointed out, under ordinary conditions substantially all of the plating system components should be relatively inert to heat and catalytic reaction, including decomposition, except the decomposable plating compound. It will be understood that the requirement of an inert atmosphere and relatively inert plating system components is applicable to the second embodiment process as well as the first. The plating process should be undertaken in an inert atmosphere to minimize decompositon of the solubilized plating compounds which readily combine with oxygen. Suitable inert atmospheres for use in the first and second embodiments of the invention are exemplified by nitrogen, hydrogen, methane, argon, helium and the like, although substantially any inert material may be used. It is significant that under minimal conditions the plating system may contain only the aluminum hydride complex and an appropriate catalyst, provided at least one of these components is a liquid at plating temperatures, since, as previously noted, a liquid medium is necessary for plating to occur. However, if both the aluminum hydride-trihydrocarbylamine complex and the catalyst are solids at the temperature selected for plating, a suitable solvent must be used. Indeed, such a solvent may even be desired for reasons which will hereinafter be developed where either or both system constituents are liquid. Useful solvents which may be utilized in this embodiment of the invention are; first, those which are relatively inert to other plating medium components; secondly, those having thermal stability up to a temperature of at least 150 0.; third, those which are solvents for either or both the complex and catalyst in event one or both of these components are solids; and last, those which are miscible with the system constituent or constituents if these are liquids at the desired operating temperature. In addition, if relatively costly solvents are to be utilized in the process, only those having a boiling point above that of the temperature selected for plating should be chosen in order to minimize loss from the plating medium. However, by selecting a relatively cheap solvent having a boiling point close to the temperature at which plating will be effected, a desirable inert atmosphere may be created by the vaporizing solvent. This technique of operation will dispense with the requirement of an external inert atmosphere, but make-up solvent should be added in order to ensure the presence of a liquid plating medium. Generally, liquid organic solvents having up to about 25 carbon atoms are preferred since they are more economical, readily available commercially and offer the necessary desirable operational features. By thermal stability it is meant that the solvent must not decompose when subjected to heat and by relatively inert it is meant that the organic material must dissolve the plating compound in a suificient concentration with substantially no reaction between the plating compound or the substrate being plated. Further, the solvent used must be compatible with the deposited aluminum coating, although it will be recognized that the products of decomposition of certain solvents may become desirably entrained in the coating to form alloyed aluminum plates exhibiting desirable physical and chemical properties.

Exemplary of useful solvents which may be used with good results in both embodiments of the invention are liquid hydrocarbons such as alkanes, aromatics, cycloalkanes, fused ring aromatics and the like. Ethers such as the alkyl, glycol, polyaromatic and similar ethers may also be utilized, as well as selected amines, and in certain instances, such compounds as fluorocarbons, silicone oils, and the like. Thus, typical specific solvents which may be used are kerosene; Primol-D (an essentially aliphatic hydrocarbon solvent); pentane; hexane; heptane; decane; octane; benzene toluene; xylene; alpha-methyl naphthalene; beta-methyl naphthalene; 1,4-dimethyl naphthalene; decahydronaphthalene; 1,2,3,4-tetrahydronaphthalene; 1- ethyl-3-methyl benzene; l-ethyl-4-methyl benzene; isopropyl benzene; 1-methyl-3-propyl benzene; 1-methyl-4-propyl benzene; cyclopentane; cyclohexane; ethylene glycol dimethyl ether; ethylene glycol diethyl ether; diethylene glycol dimethyl ether; propylene glycol dimethyl ether; diphenyl ether; anisole; propyl ether; tetrahydrofuran; trimethylamine; tributylamine; tetramethylethylene diamine; gasoline; diesel fuel; other petroleum distillates and the like.

As noted above, the use of a trialkylamine complex of aluminum hydride as the plating compound is generally preferred in the invention and this preference is characteristic of the inventive first embodiment. Further, it has been found that trialkylamine complexes containing alkyl groups having long chains may be used, so long as these complexes can be solubilized with a suitable solvent. Thus, aluminum hydride-trialkylamine complexes containing alkyl groups each having from 1 to about 10 carbon atoms are preferred for use in the first embodiment of the invention.

It was additionally previously noted that the process of this invention may be undertaken at low temperatures, that is, at temperatures between about 25 C. and about C. Accordingly, it is preferable to carry out the first embodiment procedure at a temperature of at least 25 C. in order to realize a plating rate conducive to feasible commercial operation. Therefore, a preferred feature of this embodiment is the plating technique of using a trialkylamine complex of aluminum hydride which is characterized by alkyl groups each having from 1 to about 10 carbon atoms and more preferably, from 1 to about 6 carbon atoms, to coat a selected substrate with aluminum at a temperature of from about 25 C. to about 150 C. Where plating at temperatures not substantially higher than 25 C. is desired the plating medium may be brought to this higher temperature by heating the substrate to be coated to a predetermined temperature and immersing it in the plating liquid without the application of another heat source. Of course, this procedure may be followed at still higher plating temperatures by application of auxiliary heat such as from an oil bath or resistane methods, etc., where the substrate is composed of a material which is thermally stable at this temperature. Indeed, preheating the substrate is a desirable operational procedure since it is believed that under the first embodiment reaction conditions where the catalyst is in the plating system and the substrate is not preheated, catalysis causes decomposition of the plating compound throughout the plating medium rather than only on the surfaces of the substrate. In contrast, where the substrate is preheated, plating appears to preferentially occur on the substrate surface; thus, a plating compound savings may be effected by utilizing this technique in the first embodiment of the invention.

As heretofore noted, a, preferred method of practicing the first embodiment of the invention is the addition of a solvent to the system containing a trihydrocarbylaminealuminum hydride complex and a catalyst in order to dissolve these components. Solvent addition is a desirable aspect of the operating procedure even if the complex or catalyst or both are liquids at the desired plating temperature, since it has been found that neither the complex nor the catalyst must be present in the plating system in large quantities for effective plating to occur. Thus, use of a solvent effects a substantial savings in catalyst and plating compound costs. Accordingly, still another preferred first embodiment plating procedure is characterized by using a trialkylamine complex of aluminum hydride containing alkyl groups having from 1 to about 20 carbon atoms as the plating compound, adding a suitable solvent to solubilize this compound and the catalyst, and elfecting the aluminum plating at a temperature of at least 25 C. Under these and other disclosed conditions of operation the solvent should be added to the catalyst-trialkylamine complex medium in quantities sufficient to dissolve the com plex. More preferably, the solvent should be added in quantities such that the concentration of the complex, which is most preferably a trimethylamine complex of aluminum hydride, is within the range of from about 1 to about 50 percent by weight and the catalyst is present in a concentration of at least 0001 percent by weight.

The use of a trimethylamine complex of aluminum hydride as the plating compound in the first embodiment process is desirable because this compound is easily and cheaply prepared and it is also easily dissolved in most of the common organic solvents. Other aluminum hydride-trialkylamine complexes possessing similar desirable plating characteristics are those wherein the alkyl groups each contain up to about 6 carbon atoms. Consequently, an additionally preferred feature of the first embodiment of the invention includes plating a substrate with the aid of a trialkylamine complex of aluminum hydride which contains alkyl groups each having no more than 6 carbon atoms. A compound so characterized should preferably be present in the plating system in a concentration of from about 1 to about 10 Weight percent along with at least one of the catalysts, ferric chloride, manganese dichloride, titanium tetrachloride and a titanium alkoxide, which have proved to be effective in both embodiments of the invention as heretofore noted. Of these, the most preferred catalyst is titanium tetrachloride. These and other catalysts are preferably introduced into the plating medium in relatively small amounts, that is, from about 0.1 to about 5 percent by weight. It will be recognized that the quantity of catalyst used in this and other features of the first embodiment, as well as in the hereinafter described second embodiment of the invention, should be on the low side in order to ensure use of minimal amounts of catalysts which will induce the desired aluminum deposition. Further, the solvent which is preferred in this aspect of the first embodiment technique and which forms the balance of the plating system, or solution, is one of the classes heretofore noted, and particularly, an aromatic hydrocarbon. When a plating solution so characterized is utilized in the invention, a desired temperature range within which to plate is from about 25 C. to about 150 C. It is additionally desirable to plate in the presence of an inert atmosphere, preferably nitrogen, when using the plating conditions immediately above, and particularly, where the aluminum hydride-trialkylamine complex is a trimethylamine complex of aluminum hydride and the plating temperature is within the range of from about 60 C. to about 80 C.

A highly desirable and significant result of plating with the catalyst and plating compound in a common system by means of the first embodiment procedure is the ease with which coatings of varied thickness are achieved. Effective control over thickness is thus realized by varying the plating time.

The second embodiment of the invention is characterized by a process for plating aluminum on a substrate by effecting the steps of:

(A) Coating the substrate with a catalyst capable of facilitating decomposition of a trihydrocarbylamine complex of aluminum hydride into aluminum, and

(B) Contacting the substrate in an inert atmosphere with a trihydrocarbylamine complex of aluminum hydride.

Although it is usually possible to obtain aluminum coatings of desirable thickness by effecting a single contact with the catalyst and plating compound according to steps A and B, thicker plates can easily be deposited by repeating this procedure.

As heretofore noted, preferred catalysts for use in this embodiment of the invention are ferric chloride, manganese dichloride, titanium tetrachloride and a. titanium alkoxide such as titanium isopropoxide; of these, the most preferred catalyst is titanium tetrachloride. As in the first embodiment process, these catalysts may be utilized alone or in combination, although it is desirable to use only one in any given plating operation due to the economics involved.

It is significant that the above catalysts need not be present in the liquid phase in order to be effectively contacted with the substrate to be coated. It has surprisingly been found that substrates which are exposed to catalyst vapors in step A are easily plated by subsequently carrying out step B. Thus, a simple catalyst-contacting step may be carried out where substrates are passed through the catalyst vapor at substantially any desired temperature to effect step A of the inventive second embodiment. A non-limiting example of achieving such vapor-substrate contact is the placing of iron nails in a test tube to which has been added a small drop of catalyst, which subsequently vaporizes and comes into contact with the surfaces of the nails. Aluminum plating of the nails is easily accomplished by contacting the nails with the plating solution according to the procedure of step B. A preferred catalyst for vapor coating substrates according to step A above is titanium tetrachloride. It will be appreciated that many other techniques such as spraying (both wet and dry) and splashing may be used in step A for coating the substrate to be plated according to the knowledge of those skilled in the art. It will further be recognized that the catalyst may be contacted with the substrate by such techniques as immersing the substrate to be coated in the neat catalyst or in a solution containing the catalyst and a suitable solvent. The latter procedure is preferred for reasons of economy, as heretofore noted.

Although plating compounds of the classes heretofore described may be used in this embodiment of the invention, as in the first embodiment coating technique, a preferred plating compound for use according to the second embodiment is a trialkylamine complex of aluminum hydride containing alkyl groups each having from 1 to about 10 carbon atoms. It is further desirable to solubilize the selected complex with a solvent in order to effect step B above, although this procedure is not absolutely necessary under conditions where the selected plating compound is a liquid. Of course, even where this is the case a solvent may be used and is indeed preferred, in order to conserve plating compound and thereby effect a savings in costs. It is also preferable to carry out step B at a temperature of at least 25 C. in order that the plating may occur at an appreciable rate. It will be appreciated that step A is not temperature dependent, although the same temperature as that used in step B of the plating operation may be used for convenience.

Accordingly, in a preferred combination of the second embodiment of the invention step A is effected by using at least one of the catalysts, ferric chloride, manganese dichloride, titanium tetrachloride and a titanium alkoxide exemplified by titanium isopropoxide and step B is carried out utilizing a plating compound identified as a trialkylamine complex of aluminum hydride containing alkyl groups each having from 1 to about 6 carbon atoms. This compound is preferably dissolved in a solvent selected from the classes heretofore discussed, and the whole process is subjected to a temperature of at least 25 C. to about C.

In a more particularized preferred disclosure, the second embodiment of the invention defines a process for depositing aluminum metal on a substrate by the steps of:

(A) Contacting the substrate with a first solution containing a hydrocarbon solvent and a catalyst typified by ferric chloride, manganese dichloride, titanium tetrachloride and titanium isopropoxide for a time sufiicient to allow the catalyst to adhere to the substrate, the catalyst being present in the first solution in a concentration of from about 0.001 to about percent by weight,

(B) Removing the substrate from contact with the catalyst,

(C) Allowing or causing the substrate to dry, and

(-D) Contacting the substrate in an inert atmosphere at atmospheric pressure and a temperature Within the range of from about 25 C. to about 100 C. with a second solution containing an aromatic hydrocarbon solvent and a trialkylamine complex of aluminum hydride containing alkyl groups each having from 1 to about 6 carbon atoms, the trialkylamine complex being present in the second solution in a concentration of from 1 to about 50 percent by weight.

It will be recognized that step A of the particularized process immediately above may be effected by use of catalyst vapor as well as a neat catalyst or catalyst solution. Application may be undertaken by methods known to those skilled in the art. It has been discovered that under conditions where a catalyst vapor is used in step A the procedure of step C need not be followed; thus, under these conditions the substrate can be removed from the catalyst vapor and placed directly in the plating solution according to step D with essentially no time allowed for drying. Further, where the catalyst is dissolved in a hydrocarbon solvent, this solvent may be one of the groups heretofore discussed, although it is preferably an aromatic hydrocarbon solvent, and most preferably, benzene, toluene, xylene or mixtures thereof. In this regard, it is also significant that the aromatic hydrocarbon solvent utilized in step D is also most preferably benzene, toluene, xylene or mixtures of these solvents.

In another preferred combination feature of the particularized second embodiment of the invention the catalyst is present in the first solution in a concentration of from about 0.2 to about 1 percent by weight and the aromatic hydrocarbon solvent is benzene, toluene or xylene. Further, the trialkylamine complex of aluminum hydride is preferably a trimethylamine complex of aluminum hydride which is present in the second solution in a concentration of from 1 to about 10 percent by weight, and plating is desirably effected at a temperature within the range of from about 60 C. to about 80 C. Under these process conditions the most preferred inert atmosphere for aluminum plating is nitrogen.

Following completion of the first and second embodiment plating operations the coated substrate may be allowed to cool in an inert atmosphere if a high quality, oxide-free coating is desired for subsequent application. However, since the plating temperatures of this process are relatively low, any cooling which takes place may ordinarily be achieved in air. In either case the substrate may be washed with water or other suitable solution to remove residual plating components.

The practice and advantages of this invention will become further apparent from a consideration of the following examples. It will be appreciated, however, that these examples are presented for purposes of illustration and are not intended to unduly limit the scope of this invention.

EXAMPLE I The plating compound AlH -N(CH was prepared by placing in a 1-liter flask 300 milliliters of diethyl ether and adding 53.7 grams of lithium aluminum hydride in a nitrogen atmosphere. The fiask was removed from contact with the nitrogen and fitted with an air drier, a stirrer and a Dry Ice condenser. A flask containing 107.3 grams of N(CH -HCl which was previously dried with toluene was then connected to the l-liter flask, and the amine-HCl slowly added to the diethyl ether-lithium aluminum hydride solution. 300 milliliters of benzene was then added and the diethyl ether removed under vacuum. The product benzene-AlH -N(CH plating solution was then filtered and catalysed aluminum plating was accomplished by the following procedure.

25 milliliters of the plating solution was placed in a flask, about 25 milliliters of diphenyl ether added, and benzene flashed off by heating the solution to C. in an oil bath. Essentially no aluminum plated out on the sides of the flask. A trace of titanium tetrachloride was then added and immediate hydrogen evolution occurred along with the formation of an aluminum mirror on the sides of the flask.

EXAMPLE 11 25 milliliters of the AlH -N(CH -benzene plating solution prepared according to the procedure outlined in Example I was placed in a test tube and 0.02 milliliter of titanium isopropoxide added. The mixture was slowly heated from room temperature in an oil bath. Hydrogen evolution and aluminum plating on the inside of the test tube was quite noticeable at 65 C.

EXAMPLE III The pointed end of a steel nail was scratched with sandpaper and dipped into a solution containing 0.25 milliliter of titanium tetrachloride and 50 milliliters of toluene. The nail was allowed to dry and was then dropped into a heated solution of AlH -N(CH dissolved in benzene. At a temperature of 90 C. an aluminum plate formed on the entire surface of the nail, including the area which was not sanded. Upon close examination the point of the nail was observed to be evently coated with aluminum with no surface irregularity or breaks in the plate.

EXAMPLE IV A steel nail was dipped into a beaker containing neat titanium isopropoxide, allowed to dry and then placed in a graduate cylinder containing a benzene-AlH -N(CH solution. The graduate cylinder was next placed in a nitrogen dry box at about 25 C. After a period of about 1 hours the nail was coated with a thin layer of aluminum metal and an aluminum mirror was observed on the inside of the graduate cylinder.

EXAMPLE V A 1" carbon steel cube was acid etched with nital in preparation for aluminum plating. The cube was then dipped into a solution containing 0.25 milliliter of titanium isopropoxide in 50 millilters of benzene, allowed to dry and placed in a benzene solution of AlH -N(CH at a temperature of 70 C. After 30 minutes a thin, uniform film of aluminum plate formed on the tube surfaces. Microscopic examination of the plate showed random crystal orientation in an exceptionally tight lattice. The aluminum coating was observed to be very adherent, as it resisted removal by application of Scotch tape. The dully gray matte finish was easily made shiny *by polishing.

EXAMPLE VI A test for effective solution plating catalysts was run by first preparing prospective catalysts in the following solutions:

(a) 0.50 gram of anhydrous ferric chloride in 50 milliliters of ethanol.

(b) 0.50 gram of anhydrous manganese dichloride in 50 milliliters of ethanol.

(c) 0.25 milliliter of titanium tetrachloride in 50 milliliters of benzene.

Three nails were then scratched with sandpaper and one was dipped in solution (a), another in solution (b) and the third in solution (c). The nails were allowed to dry and were then each placed in a separate test tube containing a solution of AlH-N(CH dissolved in benzene. The test tubes were heated in an oil bath to a temperature of 70 C. where they were allowed to stand for 30 minutes. Observation of the nails after this period 11 of time clearly showed that all three were well plated with an unbroken coating of aluminum.

EXAMPLE VII The procedure of Example VI was repeated using the same equipment and solutions except that titanium isopropoxide replaced the titanium tetrachloride in solution (c) and the substrates to be plated were 3 brass screws slightly etched with nitric acid. As the temperature rose, aluminum plating occurred on the brass screw dipped in solution (b) (manganese dichloride) at 60 C. Plating occurred on the other screws at a temperature of from 70 to 75 C., and an even coating was observed on all three screws, including the threads, after 30 minutes.

EXAMPLE VIII A small piece of sheet polypropylene plastic was dipped into a solution containing 0.25 cc. of titanium tetrachloride and 50 milliliters of benzene, allowed to dry and then placed into a benzene-AlH -N(CH solution having the composition of that used in Example I. The solution was heated in an oil bath to a temperature of 70 C. where it was allowed to stand for 15 minutes. An adherent aluminum plate was observed on the surfaces of the plastic sheet after this period of time.

EXAMPLE IX The procedure of Example VIII was repeated using a small piece of molded polyvinyl chloride. A strongly adherent aluminum plate formed on the PVC at a temperature of 60 to 65 C. after 15 minutes.

EXAMPLE X The procedure of Example VIII was repeated using a small piece of injection molded polyvinyl chloride. The PVC was dropped into the plating solution at a solution temperature of about 75-80 C. and left for 3-5 minutes. After this period of time the PVC was uniformily coated with a strongly adherent film of aluminum, which, upon being buffed, appeared very shiny.

EXAMPLE XI The procedure of Example VIII was repeated in all particulars using a piece of polyvinyl fluoride as the substrate to be plated. The aluminum formed a uniform and adherent coating on this substrate.

EXAMPLE XII 25 milliliters of a solution containing AlH -N(CH and benzene was placed in a flask and heated to a temperature of 68 C. in an oil bath. An iron nail was scraped with sandpaper and placed in a small vial to which had been previously added a single drop of neat titanium tetrachloride. The nail was left in the vial for a time sufficient to allow the titanium tetrachloride vapors to come into contact with it (about 10 minutes) and was then dropped into the heated plating solution. Hydrogen evolution was observed after about 2 minutes and an even, adherent aluminum coating soon appeared on the nail. The coating was smooth and resisted attempts to rub it off.

EXAMPLE XIII A single drop of titanium tetrachloride was placed in a large test tube and allowed to vaporize. After vaporization, about 25 milliliters of the AlH -N(CH -ben2ene plating solution was added to the test tube and heated in an oil bath. Hydrogen evolution was observed at a temperature of 80 C. and in about 5 minutes an aluminum mirror deposited on inside of the lower half of the test tube covered by the plating solution. The aluminum film was observed to be shiny from the outside and dull gray on the inside.

12 EXAMPLE XIV The phenylcyclohexyloctylamine complex of AlHg is prepared using a procedure similar to that noted in Example I and this compound is dissolved in octane. The plating procedure of Example I is then repeated using the xylene-phenylcyclohexyloctylamine complex plating solution, and a graphite substrate is plated with an adherent coating of aluminum.

EXAMPLE XV The procedure of Example, XII is repeated using a plating solution containing the bis-dibutylpentylarnine complex of AlH A small piece of ceramic is placed in the solution and a thin aluminum film is observed to be present on the surface of the ceramic.

EXAMPLE XVI A solution containing the didodecenylpentylamine complex of AlH and ethylene glycol dimethyl ether is prepared by a process similar to the one outlined in Example I and a Teflon cube is prepared by buffing with a slightly abrasive brush. The substrate is then placed in a graduate cyclinder to which is previously added a single drop of titanium tetrachloride. After the titanium tetrachloride vapor is contacted with the cube for about 5 minutes, the cube is dropped into a flask of the aboveprepared plating solution at a temperature of C. Plating is observed to occur in an even, adherent coating on the surfaces of the cube.

As indicated in the foregoing examples, a variety of process conditions can be effectively employed in practicing the many features present in both embodiments of this invention. Thus, the process can be operated on a continuous basis by placing various substrates on a conveyor or similar device and causing them to move through baths of plating medium, for example, where the first embodiment technique is used. Alternatively, the second embodiment process can easily be adapted to continuous plating by causing the belts to move first into contact with a catalyst and subsequently into the bath of plating liquid. Of course, these and other methods of operation, including the use of batch techniques for both of the inventive embodiments, are well within the purview of the skilled artisan.

B'y utilization of these and other techniques known to those skilled in the art, a strongly adherent aluminum coating of desired thickness may be eflected according to the first embodiment of the invention by allowing the substrate to remain in the catalyst-plating compound medium for a predetermined length of time. The same result may be achieved by use of the second embodiment technique by multiple catalyst and plating medium dips.

Further, the coating deposited by this inventive process is superior to those applied by other processes in that all surfaces of the substrate are coated uniformly, including projections, grooves, and sharp points and edges. This result is explained by investigation of the crystal morphology characteristics of the catalyst-induced aluminum 'plate. Electron microscopic examination of the crystal lattice of this coating shows that the crystal orientation is tightly packed and random rather than characterized by nucleation, which is the crystal growth mechanism characteristic of vapor coating and other previously developed techniques. Thus, the aluminum plate deposited by practicing this invention forms a tighter, less porous and more strongly adherent coating than the heretofore developed processes, since there are essentially no long cracks between crystals which frequently result from nucleated crystal growth. Crystal growth in this manner, in contrast with catalyzed crystal formation, originates from centers of nucleation, or seeds, and expands by grain growth in a certain predictable orientation. Such growth often results in a spongy, low density aluminum coating. Furthermore, the catalyzed aluminum plate has 13 been microscopically observed to possess good throwing power characteristics; that is, the coating follows all of the contours of the underlying substrate with uniform thickness and good tenacity on all surfaces, including sharp projections and edges.

Another significant limitation associated with the prior art which is overcome by this invention is the use of high temperatures. The temperatures exemplified in the above examples which may be utilized in the invention are mild and heat requirements are therefore moderate. Temperatures ranging from 25 C. to 150 C. broadly define the operable span of this invention; however, temperatures on the order of from 60 C. to 80 C. are much more preferred, as previously noted, in comparison with previously disclosed process temperatures ranging from 170 C. (a recognized minimum in the prior art) to 1800 C. and higher.

Still another advantageous feature of this invention is the utilization of catalysts in extremely low concentrations, preferably in parts per million, as heretofore noted. Thus, catalyst costs are minimal in this process while their contribution to the art of aluminum plating is maximal. It will be noted that while those examples which are characteristic of the second embodiment process recite that the catalyst on the substrate is allowed to dry before the substrate is placed in the plating medium, this is not an absolutely necessary feature of the process. The drying step is preferred because immersion of a substrate which is still wet with a catalyst or catalyst solution will cause the deposition of an aluminum coating having poor adhesion qualities.

Still another advantage inherent in this invention over prior art processes is a reduction of the problem of toxicity associated with vapor plating. By diluting the toxic plating compounds, one can ensure that workers are not exposed to concentrated quantities of these compounds.

Additional advantages of the process, such as simplicity of operation and equipment, achievement of a more efficient consumption of plating compounds, greater safety and the like, are apparent from a consideration of this invention as set forth in the specification and appended claims.

What is claimed is:

1. A process for plating aluminum metal on a substrate which comprises contacting said substrate in an inert atmosphere with a liquid phase system composed of a trihydrocarbylamine complex of aluminum hydride and a catalyst facilitating decomposition of said complex into aluminum, said catalyst being selected from the group consisting of ferric chloride and manganese dichloride.

2. The process of claim 1 wherein said complex is a trialkylamine complex of aluminum hydride containing alkyl groups each having from 1 to about 10 carbon atoms.

3. The process of claim 1 wherein said process is effected at a temperature of at least 25 C.

4. The process of claim 1 wherein:

(a) said complex is a trialkylamine complex of aluminum hydride containing alkyl groups each having from 1 to about 6 carbon atoms; and

(b) said process is effected at a temperature in the range of from about 25 C. to about 150 C.

5. The process of claim 1 wherein said complex and said catalyst are dissolved in a solvent.

6. The process of claim wherein said solvent is an aromatic hydrocarbon selected from the group consisting of benzene, toluene or xylene.

7. The process of claim 5 wherein:

(a) the concentration of said complex in said system ranges from about 1 to about 50 weight percent; and

(b) said catalyst is present in said system in a concentration of at least 0.001 percent by weight.

8. The process of claim 1 wherein said complex is a trimethylamine complex of aluminum hydride.

9. The process of claim 1 wherein said substrate is heated to further facilitate said decomposition.

10. The process of claim 1 wherein:

(a) said complex is a trialkylamine complex of aluminum hydried having alkyl groups each containing from 1 to about 6 carbon atoms and is present in said system in a concentration of from about 1 to about 10 percent by weight;

(b) said catalyst is present in said system in a concentration of from about 0.1 to about 5 percent by weight;

(c) the balance of said system is an aromatic hydrocarbon solvent for said complex and said catalyst; and

(d) said process is conducted at a temperature within the range of from about 25 C. to about 150 C.

11. The process of claim 10 wherein:

(a) said inert atmosphere is nitrogen;

(b) said complex is a trimethylamine complex of aluminum hydride; and

(c) said temperature is within the range of from about 60 C. to about C.

12. A process for plating aluminum metal on a substrate which comprises:

(a) coating said substrate with a catalyst capable of facilitating decomposition of a trihydrocarbylamine complex of aluminum hydride into aluminum selected from the group consisting of ferric chloride and manganese dichloride; and

(b) contacting said substrate in an inert atmosphere with a trihydrocarbylamine complex of aluminum hydride.

13. The process of claim 12 wherein steps (a) and (b) are repeated until a desired thickness of aluminum coating is plated on said substrate.

14. The process of claim 12 wherein said complex is a trialkylamine complex of aluminum hydride containing alkyl groups each having from 1 to about 10 carbon atoms and said complex is dissolved in a hydrocarbon solvent.

15. The process of claim 12 wherein (a) said complex is a trialkylamine complex of aluminum hydride containing alkyl groups each having from 1 to about 6 carbon atoms and is dissolved in an aromatic hydrocarbon solvent; and

(b) said contacting is affected at a temperature in the range of from about 25 C. to about 150 C.

16. The process of claim 12 wherein the coating of the catalyst on the substrate is applied thereto by exposing the substrate to vapors of the catalyst.

17. The process of claim 12 wherein the coating of the catalyst on the substrate is applied thereto by exposing the substrate to a solution of the catalyst.

18. A process for depositing aluminum metal on a substrate which comprises:

(a) contacting said substrate with a first solution containing a hydrocarbon solvent and a catalyst selected from the group consisting of ferric chloride and manganese dichloride for a time sufficient to allow said catalyst to adhere to the substrate, said catalyst being present in said first solution in a concentration of from about 0.001 to about 5 percent by weight;

(b) removing said substrate from contact with said first solution;

(0) allowing or causing said substrate to dry; and

(d) contacting said substrate in an inert atmosphere at atmospheric pressure and a temperature within the range of from about 25 C. to about C. with a second solution containing an aromatic hydrocarbon solvent and a trialkylamine complex of aluminum hydride containing alkyl groups each having from 1 to about 6 carbon atoms, said trialkylamine complex being present in said second solution in a concentration of from about 1 to about 50 percent by weight.

19. The process of claim 18 wherein:

(a) said catalyst is present in said first solution in a concentration of from about 0.2 to about 1 percent by weight;

(b) said aromatic hydrocarbon solvent is benzene, toluene or xylene;

(c) said trialkylamine complex of aluminum hydride is a trimethylamine complex of aluminum hydride and is present in said second solution in a concentration of from about 1 to about 10 weight percent;

((1) said inert atmosphere is nitrogen; and

(e) said temperature is within the range of from about 60 C. to about 80 C.

1 6 References Cited UNITED STATES PATENTS 3,462,288 8/1969 Schmidt et a1. 117-47 R X 5 3,206,326 9/1965 Whaley et a1. 117--107.2 R 3,449,144- 6/ 1969 Williams et a1. 117-47 R X RALPH S. KENDALL, Primary Examiner C. WESTON, Assistant Examiner 10 US. Cl. X.R.

117-47 A, 47 R, 107.2 R, 160 R Patent No. 5,7 5, 51 Dated D m r 5, 1972 Inventor(s) Paul Kobetz, Warren E, Becker, Albert P. Giraitis It is certified that error appears in "the above-identified patent and that said Letters Patent are hereby corrected as shown below:

if v "9 Column 1, line 22, reads "catalysts", should read catalysis Column 9, line 68, reads fiask", should read flask Column 10, line 51, reads "tube", should read cube Column 10,- line 56, reads "dully, should read dull Column l0, line 72, reads "AlHN(CH should read All-I -N(CH Column 14, line 5, reads "hydried", should read hydride Signed and sealed this 29th day of May 1973 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

